The present invention relates to a method of producing chlorine dioxide. The method comprises reacting chloric acid with hydrogen peroxide as a reducing agent.
Chlorine dioxide used in aqueous solution is of considerable commercial interest, mainly in pulp bleaching, but also in water purification, fat bleaching, removal of phenols from industrial wastes etc. It is therefore desirable to provide processes in which chlorine dioxide can be efficiently produced.
There are numerous different processes for chlorine dioxide production. Most processes in commercial use involves reaction of sodium chlorate in an acidic medium with a reducing agent such as chloride ions, methanol or sulfur dioxide. The acidity is generally provided by sulfuric acid. A drawback of the known processes is the formation of some form of sodium sulfate as a by-product which has to be removed from the reactor, either in the form of a solid saltcake or as waste acid.
Chlorine dioxide can also be produced from chloric acid (HClO.sub.3), involving the advantage that no sodium sulfate by-product is produced.
Chloric acid can be prepared by ion exchange such as described in, for example, U.S. Pat. Nos. 3,810,969 and 4,798,715, or by electrolyses of alkali metal chlorate such as described in, for example, U.S. Pat. No. 4,915,927. Chloric acid can also be prepared by treating barium chlorate with sulfuric acid and concentrating the dilute chloric acid obtained by evaporation. The disclosures of the above patents are incorporated herein by reference.
International patent application WO 91/12356 (corresponding to U.S. Pat. No. 5,084,148), the disclosure of which is incorporated herein by reference, describes a process of producing chloric acid by electrolysing alkali metal chlorate in a divided cell, alkali metal hydroxide, oxygen gas and hydrogen gas being obtained as a valuable by-products. It is said that the chloric acid can serve as a raw material in commercial processes for producing chlorine dioxide, using chloride, sulfur dioxide or methanol as a reducing agent.
International patent application WO 92/03374 discloses a process which involves partly converting alkali metal chlorate to chloric acid by electrolyses, feeding a chlorine dioxide reactor with the mixture, and recycling the chlorate to the electrochemical cell. The chlorine dioxide is produced electrochemically or with methanol as a reducing agent.
Electrochemical production of chlorine dioxide gives comparatively low current efficiency as well as chemical efficiency. Moreover, the potential for chlorine dioxide decomposition is always present and thus a hazard to sensitive and expensive electrochemical cell equipment.
A draw back of using methanol as a reducing agent is the low reaction rate at low acidities. In order to obtain high acidity, the electrolyses of chlorate to chloric acid can be run to a high degree of conversion which, however, has been found to decrease the current efficiency. Production of chlorine dioxide at low acidities requires high methanol concentration.
Another draw back of using methanol, particularly when it is used at high concentrations, is the formation of organic by-products such as formaldehyde and formic acid. Also some of the methanol leaves the reactor without having participated in the reduction, and the corresponding ether and ester are there as well. A chlorate solution containing the above impurities may cause damage to an electrochemical cell and it is therefore desirable to recrystallize and redissolve the recycling alkali metal chlorate before it enters the electrochemical cell, as described in the examples of the above mentioned WO 92/03374.